1. Technical Field
This invention relates generally to the field of wireline communications and more particularly to coupling of signal processing circuitry with a wireline communications medium.
2. Related Art
In communications systems, a communication signal may be received from a wireline. The communication signal is typically an electromagnetic signal generated to transmit information or data. The wireline may be any physical medium configured to carry a signal such as a wire, a twisted pair cable, a coaxial cable, an electrical cable, or the like. The wireline may include a plain old telephone service (POTS) line, a Digital Subscriber Line (DSL), an Asymmetric Digital Subscriber Line (ADSL), an Ethernet line, or a powerline. The wireline transmits many signals having varying characteristics. For example, signals may be within various frequency bands or of varying strengths. The wireline may also transmit a power signal which is not a communication signal, because the power signal may not transmit information or data.
The range of signal strengths depends on the strength and position of a transmitter, impedance, and noise on the wireline. For example, in a powerline, the range of signal strengths varies from the maximum allowable injected power to the noise floor. To illustrate, in the 1-30 MHz band, the maximum allowable injected power is approximately −50 dBm/Hz while the noise floor is approximately −150 dBm/Hz. This results in an overall input dynamic range of about 90 dB. Circuitry that is configured to receive a relatively large signal is typically less adept at processing small signals and vice-versa. There is, therefore, a trade-off between detecting a large dynamic range of communication signals and detection sensitivity.
FIG. 1 illustrates a communications system 100 for processing a communication signal in the prior art. In the communications system 100, a coupling unit transfers the communication signal from a wireline input 102 from to signal processing circuitry. The coupling unit may include a first filter 104 and a single ratio transformer 106. The first filter 104 and the single ratio transformer 106 isolate the signal processing circuitry and transform the voltage of the received communication signal to prevent damage to the signal processing circuitry. The single ratio transformer 106 is a transformer having a single winding ratio between a primary coil and a secondary coil. The winding ratio is the ratio of the number of windings between the primary coil and the secondary coil, and dictates the voltage gain or attenuation of the received communication signal by the single ratio transformer 106. A second filter 108 filters the signal output from the single ratio transformer 106.
A transformer having a single winding ratio, such as single ratio transformer 106, will always produce an output signal having a fixed magnitude ratio relative to the received signal. As a result, the signal processing circuitry following the transformer must be configured to receive signals of a wide range in magnitudes, or distortions may occur. Input signals that are too large may be clipped while input signals that are too small may be indistinguishable from a noise level.
To accommodate the variation in signal magnitudes as may be received from the single ratio transformer 106, some systems include variable circuitry after the single ratio transformer 106. For example, a programmable gain stage, such as a single input active programmable gain amplifier (PGA) 110, can be used to amplify or attenuate signals into a range preferred by an analog to digital converter (ADC) 114. As the programmable gain stage typically uses active components to amplify the signal, the programmable gain stage contributes to signal distortion, is limited by a noise floor, and is subject to signal magnitude offsets. Further filters, such as third filter 112, may be used to remove noise from the signal due to amplification by the PGA 110.
Typically, an Automatic Gain Control (AGC) system 116 is used to control the PGA 110 through a feedback loop 118. The ADC 114 outputs a digital signal at digital signal output 120. The digital signal may be representative of the received communication signal.